1. Field of Invention
The present invention pertains to a refrigeration process that employs a mixed refrigerant system to chill a process gas stream, and more particularly to an improved mixed refrigeration system in which a second stream, distinct from the process gas stream to be chilled, is cooled against rewarming vaporized mixed refrigerant, and then throttled and at least partially vaporized to provide duty to at least partially condense the compressed mixed refrigerant.
2. Description of the Prior Art
Mixed refrigerant systems have been well known in the industry for many decades. In these systems, multiple components are utilized in a single refrigeration system to provide refrigeration covering a wider range of temperatures, enabling one mixed refrigeration system to replace multiple pure component cascade refrigeration systems. The mixed refrigeration systems have found widespread use in base load liquid natural gas plants.
Mixed refrigerant systems have been used extensively in many applications, for example, in the chilling and liqufaction of natural gas to produce natural gas liquids or liquefied natural gas, and within the chilling and demethanization steps in an ethylene production process. The advantages of using a mixed refrigerant as compared with a one-component refrigerant system include both energy and capital savings. Energy savings can be achieved because mixed refrigerants provide a better match between process cooling curves and refrigerant evaporating curves and therefore reduce energy losses due to the required temperature driving forces. Capital savings can arise because fewer compressor stages may be needed, and they may be able to be constructed from cheaper non-cryogenic materials.
A typical mixed refrigerant system consists of a compressor of one or more stages that compresses a relatively low pressure mixed refrigerant stream to a higher pressure. This higher-pressure stream is then at least partially condensed to provide a liquid mixed refrigerant. This liquid mixed refrigerant can be subcooled before it is flashed to a lower pressure. This flashed stream is at a lower temperature than the high-pressure stream, and still contains some liquid. This liquid is vaporized at the lower temperature to provide refrigeration to a process stream or streams. Typically the process stream is chilled in multiple stages, with refrigeration to each of these stages provided by vaporizing or reheating the mixed refrigerant. The vaporized mixed refrigerant is typically reheated to some optimum temperature against the process stream to extract the maximum amount of refrigeration from the mixed refrigerant. The mixed refrigerant stream is typically fully vaporized by heat exchange with the process stream or streams.
Much effort has gone into optimizing the design of these mixed refrigerant systems, with particular attention being paid to how the compressed and (at least partially) condensed mixed refrigerant is handled and brought into heat exchange with the process stream that is to be chilled. Multiple patent and literature citations attest to the variety of mixed refrigerant system designs that alter, for example, the number of vaporization pressure levels, the treatment of gas and liquid mixed refrigerant streams in partially condensed systems, the number of vaporization steps, and the integration of these parameters with the process stream to be chilled.
In a mixed refrigerant system one of the major duties is the condensing of the compressed mixed refrigerant. This will typically be done at least partially against cooling water or other ambient cooling stream. In addition a separate propane or propylene refrigeration system could also be used to provide some of the condensing duty for the mixed refrigerant. Other cold process streams that need to be warmed can also be used to condense the mixed refrigerant stream. In addition, some of the condensing duty is typically provided by vaporization of the low-pressure mixed refrigerant itself. Likewise, more refrigeration can be recovered from the vaporized mixed refrigerant stream by reheating it against a process stream that needs to be cooled. Warming the vaporized mixed refrigerant before returning it to the compressor may also allow the mixed refrigerant compressor to be constructed out of a cheaper non-cryogenic material, resulting in capital savings.
We have found that the process can be simplified and energy savings realized when a single stream is used to both reheat the vaporized mixed refrigerant stream and provide condensation duty to the compressed mixed refrigerant. A key aspect of this invention is that a relatively high-pressure liquid stream is subcooled against the reheating vaporized mixed refrigerant stream, then it is flashed to a lower pressure and vaporized to provide the condensing duty to the compressed mixed refrigerant stream.
It has never been suggested that the mixed refrigerant vapor can be reheated against a stream other than a process stream to be chilled. McCue et al., U.S. Pat. No. 5,768,913 discloses a process and system for providing cooling for a gas separation process wherein the refrigerant is obtained from the system process fluid and after serving as a refrigerant is returned to the process side for separation into product. The patent teaches subcooling a liquid process stream and then throttling the subcooled liquid to provide refrigeration to the process. At least some of the subcooled liquid is throttled to provide the subcooling duty.
The process of this invention is an improvement over prior art processes in that it reduces the energy required for operation of the mixed refrigeration system. It recognizes the beneficial synergy between the required heating and cooling duties of a mixed refrigerant system and the heating duty required in vaporizing a liquid stream. Furthermore the process of this invention utilizes the heating duty of the mixed refrigeration system in a more desirable way than prior art processes. Subcooling the liquid before vaporization makes more of its latent heat of vaporization available at a lower temperature for mixed refrigerant condensation, and this subcooling is itself useful in the mixed refrigerant system. Moreover it has been found that this invention is particularly beneficial when the refrigeration system utilizes a mixed refrigerant and the liquid stream is not a single component—it boils over a relatively large temperature range.
An improved process is disclosed for the design of a mixed refrigeration system for chilling a process gas. The improvement comprises utilizing a second stream, distinct from the process gas to be chilled, to provide condensing duty to the compressed mixed refrigerant stream. The second stream is first cooled against a rewarming vaporized mixed refrigerant stream, and then throttled and at least partially vaporized to provide duty to at least partially condense a compressed mixed refrigerant stream.